Managing read requests from multiple requestors

ABSTRACT

Techniques are disclosed for managing data requests from multiple requestors. According to one implementation, when a new data request is received, a determination is made as to whether a companion relationship should be established between the new data request and an existing data request. Such a companion relationship may be appropriate under certain conditions. If a companion relationship is established between the new data request and an existing data request, then when data is returned for one request, it is used to satisfy the other request as well. This helps to reduce the number of data accesses that need to be made to a data storage, which in turn enables system efficiency to be improved.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/537,780 filed Aug. 7, 2009, which is a divisional of U.S. patentapplication Ser. No. 11/069,735 filed Feb. 28, 2005, now U.S. Pat. No.7,577,763. Both of these prior applications are incorporated herein byreference as if fully set forth herein.

FIELD OF THE INVENTION

The present invention relates to performing I/O operations directed todata managed by a file system.

BACKGROUND

Frequently, access to electronic information stored on storage devicesis managed through a file system. Often, the stored information isphysically or logically divided into blocks. For example, a storagedevice may logically divide data into 1K blocks. Thus, a file thatincludes 2K bytes may include a first block of data corresponds to theaddress range 0 to 1023, and a second block of data corresponds to theaddress range 1024-2047.

When clients access data managed by a file system, I/O requests are sentto the file system to perform the I/O operations. The data that isspecified, by the client, as the target of a requested I/O operation isreferred to herein as the “target chunk”. When the boundaries of thetarget chunk coincide with the boundaries of the blocks that contain thetarget chunk, then the I/O operation is referred to as an “aligned” I/Ooperation. On the other hand, if the boundaries of the target chunk donot coincide with the boundaries of the blocks that contain the targetchunk, then the I/O operation is referred to as an “unaligned” I/Ooperation.

Frequently, storage devices and/or file systems are designed to performaligned I/O operations more efficiently than unaligned I/O operations.Consequently, retrieving an entire block may be more efficient thanretrieving a target chunk that is only a subset of the block. Similarly,retrieving two blocks may be more efficient than retrieving a targetchunk that spans but does not fully include the two blocks.

Unfortunately, the I/O operations required by clients are not alwaysaligned, and it would place an undue burden on client developers torequire clients to be designed to only request aligned I/O operations.

Other types of inefficiencies may occur if I/O operations are notmanaged intelligently. For example, if the read requests issued to thefile system by one application are not coordinated with the readrequests issued by other applications, then the file system may, forexample, have to retrieve the same block many times in succession. Inaddition, if applications are responsible for directly sending their ownread requests to the file system, then the timing of read requests thathave deadlines may have to be handled by the applications themselves,making the applications more complex than desired.

The approaches described in this section are approaches that could bepursued, but not necessarily approaches that have been previouslyconceived or pursued. Therefore, unless otherwise indicated, it shouldnot be assumed that any of the approaches described in this sectionqualify as prior art merely by virtue of their inclusion in thissection.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a block diagram of a system in which a read schedulermaintains an in-flight list and request queue, according to anembodiment of the invention;

FIG. 2 is a block diagram of a read scheduler that expands the range ofrequests so that the requests conform to block boundaries, and cachespartially-read blocks, according to an embodiment of the invention; and

FIG. 3 is a block diagram of a computer system on which embodiments ofthe invention may be implemented.

DETAILED DESCRIPTION

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. For example, scenarios presentedhereafter use 1K for the size of blocks and pages, thereby simplifyingthe examples. However, the size of storage blocks and memory pages mayvary greatly from implementation to implementation, and will frequentlybe larger (e.g. 4K or 32K) than 1K. It will be apparent, however, thatthe present invention may be practiced without these specific details.In other instances, well-known structures and devices are shown in blockdiagram form in order to avoid unnecessarily obscuring the presentinvention.

Overview

Techniques are described herein for expanding the range of data targetedin I/O requests made by clients, so that the expanded range results inaligned I/O operations within the file system. Data that is included inthe expanded range, but was not actually requested by the client, istrimmed off the data chunk returned by the file system, so that theclient receives only the data required by the client. The blocks thatcontain the partially-read data are cached, so that they can be providedto the clients in response to subsequent I/O requests, without having toretrieve the blocks again from the file system.

Techniques are also provided for tracking requests from multiple clientsusing a single request queue and in-flight list. When new read requestsare received from clients, the contents of the request queue andin-flight list are inspected to determine whether the new request iscovered by a previously received request.

System Overview

Referring to FIG. 1, it is a block diagram that illustrates a systemconfigured according to an embodiment of the invention. Specifically, aplurality of clients 104 make I/O requests to a file system 120 via anintermediary, referred to herein as a read scheduler 112. Read scheduler112 then coordinates with file system 120 to provide the clients 104with the data they require in a manner that conforms to any deadlinesspecified by the clients 104. Media handler 102 is an example of a typeof client that requires a significant amount of I/O activity, duringactivities such as the playback or editing of digital audio and/or videofiles.

When media handler 102 requires data from the file system 120 (e.g. thenext segment of a video), media handler 102 allocates a buffer 107 toreceive the data, and requests the data from the appropriate scheduleddata handler. Different scheduled data handlers may provide data fordifferent types of media. For example, scheduled data handler 108 may beused to schedule reads for video data, while scheduled data handler 108is used to schedule reads for audio data.

For the purpose of illustration, it shall be assumed that media handler102 sends a request to scheduled data handler 106. Scheduled datahandler 106 then sends the request to read scheduler 112. According toone embodiment, scheduled data handler 106 sends request to readscheduler 112 by obtaining a mutex that governs access to a new requestqueue 118, placing the request on the new request queue 118, releasingthe mutex, and sending signal to read scheduler 112.

In many situations, such read requests not only indicate the targetchunk of data required by the client, but also indicate a deadline bywhich the client expects the data to be in the buffer. Using suchdeadline-based requests increases the flexibility of the system byallowing the requests to be handled asynchronously, while still ensuringthat the data will be available to the clients when it is needed.

Many scheduled data handlers, servicing many clients, may place requestson the new request queue 118 of read scheduler 112. Under somesituations, many of those several clients may even be reading data fromthe same file. For example, during the playback of a movie, a videohandler may be issuing read requests for the video data in a movie file,while a sound handler is concurrently issuing read requests for theaudio data in the same file.

To track the requests, read scheduler 112 maintains the new requestqueue 118, a scheduled request queue 116, and an in-flight list 114.When the read scheduler 112 becomes aware of newly arrived requests onthe new request queue 118, the read scheduler 112 moves the entries fromthe new request queue 118 onto the scheduled request queue 116. Thescheduled request queue 116 includes an ordered list of entries for I/Orequests that (1) have been received from clients, but (2) have not yetbeen passed to the file system 120. According to one embodiment, theentries in the scheduled request queue 116 are ordered according todeadline and position in file, as shall be described in greater detailhereafter. The in-flight list 114 includes entries for requests that (1)have been passed to the file system 120, but (2) have not yet been fullyprovided to the client. How read scheduler 112 maintains thesestructures shall be described in greater detail hereafter.

The Request Queue

When read scheduler 112 receives a read request from a client, therequest typically specifies (1) a file, (2) an offset into the file, (3)an amount of data to retrieve, and (4) a deadline by which to performthe read operation. Such requests are moved from the new request queue118 into the scheduled request queue 116 in an order that is based onthe corresponding deadline, where the request with the nearest deadlineis at the head of the scheduled request queue 116 (the “head request)and the request with the furthest deadline is at the tail of thescheduled request queue 116.

To the extent that multiple requests specify the same deadline, therelative order of those requests on the scheduled request queue 116 isbased on the file offset order of the requests. For example, assume thatthe scheduled request queue 116 already includes a request associatedwith time T1 and offset 512. If a second request associated with time T1arrives, the second request may be placed ahead of the existing requestif the second request is associated with offset 256, or after theexisting request if the second request is associated with the offset900.

When the current time is within some threshold of the deadline of thehead request, the read scheduler 112 sends to the file system 120requests for the read operations required by the head request. Thespecific read operations requested by the read scheduler 112 may beadjusted to achieve block-aligned and page-aligned I/O, as shall bedescribed in greater detail hereafter.

In response to sending to the file system 120 the I/O requests for thehead request, the head request is moved from the scheduled request queue116 to the in-flight list 114. The read scheduler 112 then checks thedeadline associated with the new head request. If the deadline for thenew head request is also within the threshold, it is processed in a likemanner. This process continues until either the scheduled request queue116 is emptied, or the read scheduler 112 encounters a new head requestwhose deadline is not within the threshold of the current time. Becauserequests with the same deadline are ordered based on file offset, andthe read scheduler 112 processes the requests based on their orderwithin the scheduled request queue 116, requests associated with thesame deadline are submitted to the file system 120 in an order that isdictated by their file offset.

According to one embodiment, a request can be promoted within thescheduled request queue 116 ahead of its deadline order under certaincircumstances. For example, a later request may be promoted to be neareranother, higher-ranked request based on the fact that the later requestis for I/O on the same file as the higher-ranked request.

The In-Flight List

The in-flight list is a list of the requests that have been requested ofthe file system 120, but not yet fulfilled by the file system 120. Whena request is fulfilled by the file system 120, and the necessary datahas been provided to the client that requested the data, the request isremoved from the in-flight list.

As shall be described in greater detail hereafter, to achieveblock-aligned and page-aligned read operations, a request for a singletarget chunk may actually result in three I/O requests issued by theread scheduler 112 to the file system 120. According to one embodiment,a request is not removed from the in-flight list until all of thecorresponding I/O requests to the file system 120 have been completed.

Conservation of Computing Resources

According to one embodiment, read scheduler 112 is implemented as athread that executes separate from the clients that are serviced by theread scheduler 112. In one embodiment, the separate thread of the readscheduler 112 is given a higher priority than the client threads, toensure that I/O based deadlines are not jeopardized by heavy processorusage of the client threads.

However, there may be periods during which read scheduler 112 has littleto do. During these periods, continuing to execute the read scheduler112 would waste computing resources, particularly if the read scheduler112 is assigned high priority relative to the clients. Therefore,according to one embodiment, read scheduler 112 is configured to stay inan inactive state until an event occurs that causes read scheduler 112to “wake up” and perform some action.

In one embodiment, the read scheduler 112 wakes up when any of thefollowing events occur: (1) a new request arrives from a client, (2) arequested read operation is completed by the file system 120, and (3) atimer elapses.

According to one embodiment, before the read scheduler 112 goesinactive, the read scheduler 112 sets the timer based on the deadlineassociated with the head request. The timer is set such that, if noother event wakes the read scheduler 112 in the meantime, the timer willwake the read scheduler 112 in time for the read scheduler 112 toservice the head request in a manner that meets the deadline associatedwith the head request. If there is no head request (i.e. the scheduledrequest queue 116 is empty), then the read scheduler 112 may set thetimer to a point in the distant future (virtually forever).

When the read scheduler 112 becomes active, the read scheduler proceedsthrough the following stages:

(1) issues I/O requests to the file system for any requests on thescheduled request queue whose deadline is within a certain threshold,

(2) responds to any I/O operations that have been completed by the filesystem 120,

(3) moves any requests on the new request queue 118 to the appropriatelocations on the scheduled request queue 116,

(4) sets the wake-up time of the timer based on the deadline associatedwith the head request, (or if there is no head request, the wake-up timeis set to the distant future) and

(5) goes to sleep.

According to one embodiment, the read scheduler 112 does not go to sleepuntil all of the necessary file requests have been issued, all of thecompleted I/O operations have been handled, and all of the new requestshave been moved to scheduled request queue 116. For example, assume thatthe read scheduler 112 has processed all previously-completed I/Ooperations and is about to go to sleep. At this point, if the filesystem completes another I/O operation, then the read scheduler 112 willhandle the newly completed I/O operation prior to going to sleep. In oneembodiment, the read scheduler 112 repeatedly performs stages (1) to(3), and only moves to stages (4) and (5) after an iteration in whichthere was nothing to do in stages (1) to (3).

Adjusting I/O Requests to Achieve Block Aligned I/O Operations

FIG. 2 is a block diagram that illustrates how read scheduler 112adjusts I/O requests before the requests are issued to file system 120.In the illustrated example, persistent storage 122 includes four blocksB1, B2, B3 and B4. Read scheduler 112 receives a read request for atarget chunk 210 that spans but does not entirely include the blocks B1,B2 and B3. Specifically, the target chunk 210 specified in the requestis for the data that falls in the range A-B, which includes some ofblock B1, all of block B2, and some of block B3.

When read scheduler 112 receives the request, read scheduler 112 expandsthe range specified in the request in a way that results in ablock-aligned I/O operation. Specifically, if the lower boundary of therequested range does not coincide with a block boundary, then the lowerboundary of the requested range is adjusted down to the nearest blockboundary. Conversely, if the upper boundary of the requested range doesnot coincide with a block boundary, then the upper boundary of therequested range is adjusted up to the nearest block boundary. In thepresent example, A would be adjusted down to T, and B would be adjustedup to Y. Therefore, the adjusted range would be T-Y, which exactlyincludes blocks B1, B2 and B3.

The requests that are sent from read scheduler 112 to file system 120cover all data in the adjusted range T-Y. In response to the requests,file system 120 performs aligned I/O operations to read blocks B1, B2and B3 and return the blocks requested by the read scheduler 112. Beforeproviding the data from blocks B1 and B3 to the client, read scheduler112 trims the data so that the client does not receive the portions ofblocks B1 and B3 that do not fall in the originally requested range A-B.As shall be described in greater detail hereafter, the process oftrimming may involve having the file system 120 copy the blocks B1 andB3 into a buffer maintained by the read scheduler 112, and then havingthe read scheduler 112 copy the appropriate portions of those blocksinto the client's buffer. In the present example, before the data isreturned to the client, data from the range T-A is trimmed off B1, anddata from the range B-Y is trimmed off B3. Thus, even though the I/Ooperations performed by file system 120 involve some data that was notrequested by the client, the client only receives the data that wasactually requested by the client.

The “trimming” of the retrieved data may be achieved using a variety oftechniques. The present invention is not limited to any particulartechnique. However, various techniques for performing the “trimming”shall be described in greater detail hereafter.

The Partially-Read Block Cache

In the example described above, blocks B1 and B3 contained portions that(1) where retrieved from the file system 120, but (2) were not requestedby the client. Such blocks are referred to herein as “partially-read”blocks.

It is not uncommon for a read operation that involves one portion of ablock to be followed by a read operation that involves some or all ofthe data in the remainder of the block. Thus, in the example givenabove, there may be high likelihood that the request for target chunk210 (range A-B) may be followed by a request for a target chuck 220(involving range B-C).

If block B3 was retrieved by read scheduler 112 in response to therequest for target chunk 210, then it would be inefficient to turnaround and again retrieve block B3 from persistent storage for asubsequent read operation that involves target chunk 220. To avoid theneed to re-retrieve the same block in rapid succession, read scheduler112 includes a partially-read block cache 200.

When an I/O operation performed by read scheduler 112 results inretrieval of one or more partially-read blocks, read scheduler 112stores the partially-read blocks in partially-read block cache 200. Whenread scheduler 112 receives a request for a target chunk, read scheduler112 determines whether some or all of the target chunk resides in blocksstored in partially-read block cache 200. To the extent that the targetchunk resides in blocks within the partially-read block cache 200, thedata is returned using the cached blocks. The I/O request sent by readscheduler 112 to file system 120 is adjusted to exclude any blocks thatare already available in the partially-read block cache 200.

For example, assume that blocks B1 and B3 are stored in cache 200because of the prior I/O operation involving target chunk 210. Assumethat a subsequent request arrives for the target chunk 220 thatcorresponds to the range B-C. To service this request, the readscheduler 112 would provide data in the range B-Y from the cached datablock B3. Read scheduler 112 would then adjust the request to the rangeY-C (to exclude the block that was already in cache).

After the adjustments to exclude cached blocks, the new range (Y-C) isadjusted (as described in the previous section) to achieve an alignedI/O operation. In the present example, Y would not be adjusted toachieve aligned I/O, since Y already coincides with a block boundary.However, C would be adjusted up to the block boundary Z of block B4.Thus, the I/O request sent from read scheduler 112 to file system 120would be for block B4 (e.g. range Y-Z).

When read scheduler 112 receives block B4 from file system 120, readscheduler 112 provides data in the range Y-C from block B4 to theclient, and stores block B4 as a partially-read block in partially-readblock cache 200. Thus, the client is provided data from the requestedrange B-C with only a single, one-block, aligned I/O operation.

Trimming Techniques

As explained above, in response to a client's request, the readscheduler 112 may retrieve more data from the file system 120 than whatthe client actually requested. Thus, the retrieved data must beeffectively “trimmed” before it is provided to the client.

According to one embodiment, trimming the file system output involvestransferring the partially-read blocks separately from, and to differentlocations than, the “fully-read” blocks. For example, consider thescenario described above where a client requests the target chunkassociated with the boundaries A-B. In this scenario, the read scheduler112 requests data blocks B1, B2 and B3. Of these three data blocks, B1and B3 are partially-read blocks and B2 is a fully-read block.

According to one embodiment, read scheduler 112 actually sends three I/Orequests to file system 120: one for block B1, one for the contiguousarray of fully-read blocks between the partially-read blocks (in thiscase, B2), and then one for block B3. The I/O request for eachpartially-read block copies the partially-read block into thepartially-read block cache 200 of read scheduler 112. The I/O requestfor each contiguous array of fully-read blocks copies the continuousarray of fully-read blocks directly into a buffer allocated for theclient (e.g. buffer 107 of media handler 102).

At this point, the client has block B2 and read scheduler 112 has cachedblocks B1 and B3. The client still needs to be provided a portion of B1and a portion of B3. Read scheduler 112 may then provide to the clientthe necessary portions of B1 and B3 based on the cached copies of B1 andB3.

Page-Aligned I/O Operations

As mentioned above, file system operations are made more efficient bydividing files into blocks, and performing block-aligned I/O operations.Similarly, within volatile memory, memory management is made moreefficient by dividing volatile memory into pages, and performingpage-aligned memory management operations. For example, assume that aclient has a 10K buffer that consists of ten 1K pages. Assume that a 1Kdata block is to be copied into the buffer. Under these circumstances,it is much more efficient to copy the 1K data block exactly into one ofthe 1K pages, than to copy the 1K data block into the buffer at anoffset that spans two of the pages. Specifically, if the 1K block iscopied at an offset within the client's buffer that aligns with thevolatile memory page boundaries, then the file system 120 may be able todirectly copy the 1K block from persistent storage 122 into the client'sbuffer. In contrast, if the 1K block must be copied at an offset withinthe client's buffer that does not align with the volatile memory pageboundaries, then the file system 120 may have to make an intermediarycopy of the 1K block before copying the block into the client's buffer.

For example, consider again the scenario in which the client requiresthe target chunk 210 associated with the range A-B. Assume further thatA-B is 2K of data, and that the client has allocated a buffer of two 1Kpages (P1 and P2) to store the data. In this scenario, the file systemwould copy block B2 into an intermediate buffer. Then, from theintermediate buffer, the first portion of B2 would be copied into thesecond half of P1, and the second portion of B2 to the first half P2.Blocks B1 and B2 would be copied into Partially-read block cache 200.Then, A-W would be copied from the cached B1 into the first half of P1,and X-B would be copied from the cached B3 into the second half of P2.Thus, even though blocks B1, B2 and B3 were read from persistent storage122 in a block-aligned read operation, copying 2K of data (A-B) into theclient's 2K buffer incurs the overhead of several unaligned memory copyoperations.

Techniques are provided for reducing the number of unaligned memory copyoperations that would be incurred in such scenarios. According to onetechnique, before allocating a buffer for a read operation, the clientcommunicates to the read scheduler 112 information about the targetchunk. The client may communicate, for example, the offset and size ofthe target chunk. Based on this information, the read scheduler 112determines the adjusted boundaries, as described above. The readscheduler 112 then communicates back to the client the size of thebuffer required to store the data encompassed by the adjustedboundaries. In addition, the read scheduler 112 indicates to the clientthe offset, within the client's buffer, at which the target chunk willstart. The client then allocates a buffer whose size is dictated by theadjusted boundaries, and then sends the read request to the readscheduler 112.

In response to the request, the read scheduler 112 sends a request tothe file system 120 based on the adjusted boundaries. The fully-readblocks encompassed by the adjusted boundaries can be copied by the filesystem 120 directly into the client's buffer in a page-aligned copyoperation. Any partially-read blocks are copied into the partially-readblock cache 200 in anticipation of future requests, and the appropriateportions of the partially-read blocks are copied into the client'sbuffer.

After these page-aligned copy operations, a portion of the client'sbuffer will contain data other than the target chunk. However, theexistence of such data does not interfere with the client's operationbecause the client has been provided the offset, into the client's ownbuffer, at which data of the requested target chunk begins.

In the example given above, the client allocates a buffer based on theadjusted boundaries. As a result, both the head of the buffer and thetail of the buffer may contain space that is not used to store thetarget chunk. Alternatively, the client may be instructed to allocate abuffer that is the size of the target chunk plus a front offset, wherethe front offset is the offset of the start of the target chunk withinthe storage block that contains the start of the target chunk. Forexample, the buffer allocated to store A-B may be the size of A-B plusthe offset of A within B1. When a buffer size is selected in thismanner, the start of the client buffer will still include some unusedspace, but the end of the client buffer will end exactly at the end ofthe target chunk.

Example of Page-Aligned Copy Operation

For the purpose of illustrating the technique, described above, forperforming a page-aligned copy operation, consider again the scenario inwhich a client desires the target chunk 210 defined by the boundariesA-B. Assume that the block and page sizes are 1K, and that A correspondsto an offset of 512, and B corresponds to an offset of 2560. Prior toallocating a buffer to receive the target chunk 210, the clientcommunicates the boundaries 512 and 2560 to the read scheduler 112. Theread scheduler 112 determines that, to achieve a block-aligned readoperation, the boundaries need to be expanded to T-Y (i.e. 0 and 3072).The 0-3072 boundaries encompass exactly three blocks, so the client willneed a buffer that is large enough to store three blocks (even thoughthe client only desires 2K of data). The 3K buffer size is communicatedfrom the read scheduler 112 to the client, and the client allocates abuffer that is 3K in size.

In addition to the buffer size, the read scheduler 112 communicates tothe client the offset, within the buffer, at which the data of thetarget chunk 210 will begin. In the present example, point A is 512bytes into block B1. Under these circumstances, the read scheduler 112would communicate to the client that the target chunk 210 will begin 512bytes into the buffer.

After allocating the buffer, the client sends a request for the targetchunk 210 to read scheduler 112, identifying the buffer that wasallocated to receive the data. Read scheduler 112 issues the appropriaterequests to the file system 120, and file system 120 responds bydirectly loading block B2 into the specified buffer (at offset 1024),and directly loading blocks B1 and B3 into partially-read block cache200. The appropriate portions of blocks B1 and B3 are then copied to theappropriate locations in the client's buffer. Specifically, section A-Wof B1 is copied into the client's buffer beginning at offset 512.Section X-B of block B3 is copied into the client's buffer beginning atoffset 2048. The client may then read target chunk 210 out of itsbuffer, starting at the offset previously indicated by the readscheduler 112 (i.e. the 512 byte offset).

As mentioned above, the size of the client's buffer may alternatively bebased on the size of the target data plus a front offset. In the presentexample, the buffer size would be established based on the size of range512 and 2560 plus the front offset 512. Thus, the client would allocatea buffer of 2560 bytes, rather than 3K bytes. All of the memoryoperations would be the same as described above, except that there wouldbe no unused space at the end of the client's buffer.

Handling Overlapping Requests

According to one embodiment, as the read scheduler 112 moves requestsfrom the new request queue 118 to the scheduled request queue 116, theread scheduler 112 expands the requests to achieve block aligned andpage aligned I/O operations. Thus, as the request for target chunk 210is moved from the new request queue 118 to the scheduled request queue116, the read scheduler 112 expands the request to encompass three I/Ooperations: reading block B1 into partially-read block cache 200,reading block B2 into client buffer 107, and reading block B3 intopartially-read block cache 200.

It is possible that one or more of the I/O operations required by anewly-received request is identical to or overlapping with I/Ooperations required by other requests already received by read scheduler112. According to one embodiment, read scheduler 112 looks for suchoverlaps and, when such overlaps occur, establishes a “companion”relationship to avoid redundant retrieval of the same data.

Specifically, in one embodiment, a first request is established as acompanion to a second request if (1) the data required by first requestis a proper subset of the data required by the second request (includingthe situation where both requests require exactly the same data), and(2) the deadline associated with the first request is not earlier thanthe deadline associated with the second request. When these twoconditions are satisfied, the first request does not have a separatelypositioned entry in the scheduled request queue 116. Rather, the firstrequest is identified as a companion of the second request, and isfulfilled at the time that the second request is fulfilled.

Finding Companions in the Scheduled Request Queue

According to one embodiment, when a new request is to be placed on thescheduled request queue 116, a scan of the scheduled request queue 116is made to determine whether (1) the new request qualifies as acompanion to any previously-received requests with the same or earlierdeadlines, and/or (2) any previously-received requests with the same orlater deadlines qualify as companions to the new request. If the former,then the new request is added as a companion to the previously-receivedrequest. If the latter, then an entry for the new request is added tothe scheduled request queue 116 based on the deadline associated withthe new request, and the previously-received request that qualifies as acompanion is converted into a companion of the new request (andtherefore will not have a separately scheduled entry on the scheduledrequest queue 116).

Because read scheduler 112 maintains a global scheduled request queue116 that potentially includes the requests of many data handlers, thelikelihood that a request will have companion relationships issignificantly higher than situations, for example, where each datahandler manages its own request queue. For example, assume that mediahandler 102 is playing a movie, issuing requests to read video data toscheduled data handler 106, and issuing requests to read audio data toscheduled data handler 108. If the audio is interleaved with the videowithin the movie file, there is a relatively high likelihood that atleast some of the audio I/O requests from scheduled data handler 108will qualify as companions to the video I/O requests issued by scheduleddata handler 106.

Finding Companions in the In-Flight List

According to one embodiment, prior to checking for companionrelationships in the scheduled request queue 116, a scan of thein-flight list 114 is made to determine whether the new requestqualifies as a companion to any requests in the in-flight list. If thenew request qualifies as a companion to any requests in the in-flightlist, then the new request is added as a companion to the appropriaterequest in the in-flight list 114. Under these circumstances, the readscheduler 112 does not even need to put an entry for the new request onthe scheduled request queue 116. When the data arrives for thepreviously-received request in the in-flight list 114, the data is usedto fulfill both the previously-received request and the new request.

Incoming Request Processing Example

Using the techniques described above, various operations anddeterminations are performed by read scheduler 112 to process anewly-arrived request. According to one embodiment, the read scheduler112 handles a new request according to the following sequence:

(1) the range of data required by a new request is adjusted to achieveblock and page aligned I/O operations;

(2) the in-flight list is checked to see if the new request qualifies asa companion to an in-flight request;

(3) if the new request is not a companion to any in-flight request, thenstarting at the head of the scheduled request queue, scan the scheduledrequest queue to see if the new request qualifies as a companion to arequest with an earlier deadline;

(4) if the new request does not qualify as a companion to a request withan earlier deadline, then insert an entry for the new request into thescheduled request queue; and

(5) scan the remainder of the scheduled request queue for entries withlater deadlines to see if any entries with later deadlines qualify as acompanion to the new request.

This sequence of processing a new request is merely exemplary. Theactual sequence in which the read scheduler 112 handles newly-arrivedrequests may vary from implementation to implementation.

Hardware Overview

FIG. 3 is a block diagram that illustrates a computer system 300 uponwhich an embodiment of the invention may be implemented. Computer system300 includes a bus 302 or other communication mechanism forcommunicating information, and a processor 304 coupled with bus 302 forprocessing information. Computer system 300 also includes a main memory306, such as a random access memory (RAM) or other dynamic storagedevice, coupled to bus 302 for storing information and instructions tobe executed by processor 304. Main memory 306 also may be used forstoring temporary variables or other intermediate information duringexecution of instructions to be executed by processor 304. Computersystem 300 further includes a read only memory (ROM) 308 or other staticstorage device coupled to bus 302 for storing static information andinstructions for processor 304. A storage device 310, such as a magneticdisk or optical disk, is provided and coupled to bus 302 for storinginformation and instructions.

Computer system 300 may be coupled via bus 302 to a display 312, such asa cathode ray tube (CRT), for displaying information to a computer user.An input device 314, including alphanumeric and other keys, is coupledto bus 302 for communicating information and command selections toprocessor 304. Another type of user input device is cursor control 316,such as a mouse, a trackball, or cursor direction keys for communicatingdirection information and command selections to processor 304 and forcontrolling cursor movement on display 312. This input device typicallyhas two degrees of freedom in two axes, a first axis (e.g., x) and asecond axis (e.g., y), that allows the device to specify positions in aplane.

The invention is related to the use of computer system 300 forimplementing the techniques described herein. According to oneembodiment of the invention, those techniques are performed by computersystem 300 in response to processor 304 executing one or more sequencesof one or more instructions contained in main memory 306. Suchinstructions may be read into main memory 306 from anothermachine-readable medium, such as storage device 310. Execution of thesequences of instructions contained in main memory 306 causes processor304 to perform the process steps described herein. In alternativeembodiments, hard-wired circuitry may be used in place of or incombination with software instructions to implement the invention. Thus,embodiments of the invention are not limited to any specific combinationof hardware circuitry and software.

The term “machine-readable medium” as used herein refers to any mediumthat participates in providing data that causes a machine to operationin a specific fashion. In an embodiment implemented using computersystem 300, various machine-readable media are involved, for example, inproviding instructions to processor 304 for execution. Such a medium maytake many forms, including but not limited to, non-volatile media,volatile media, and transmission media. Non-volatile media includes, forexample, optical or magnetic disks, such as storage device 310. Volatilemedia includes dynamic memory, such as main memory 306. Transmissionmedia includes coaxial cables, copper wire and fiber optics, includingthe wires that comprise bus 302. Transmission media can also take theform of acoustic or light waves, such as those generated duringradio-wave and infra-red data communications.

Common forms of machine-readable media include, for example, a floppydisk, a flexible disk, hard disk, magnetic tape, or any other magneticmedium, a CD-ROM, any other optical medium, punchcards, papertape, anyother physical medium with patterns of holes, a RAM, a PROM, and EPROM,a FLASH-EPROM, any other memory chip or cartridge, a carrier wave asdescribed hereinafter, or any other medium from which a computer canread.

Various forms of machine-readable media may be involved in carrying oneor more sequences of one or more instructions to processor 304 forexecution. For example, the instructions may initially be carried on amagnetic disk of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to computer system 300 canreceive the data on the telephone line and use an infra-red transmitterto convert the data to an infra-red signal. An infra-red detector canreceive the data carried in the infra-red signal and appropriatecircuitry can place the data on bus 302. Bus 302 carries the data tomain memory 306, from which processor 304 retrieves and executes theinstructions. The instructions received by main memory 306 mayoptionally be stored on storage device 310 either before or afterexecution by processor 304.

Computer system 300 also includes a communication interface 318 coupledto bus 302. Communication interface 318 provides a two-way datacommunication coupling to a network link 320 that is connected to alocal network 322. For example, communication interface 318 may be anintegrated services digital network (ISDN) card or a modem to provide adata communication connection to a corresponding type of telephone line.As another example, communication interface 318 may be a local areanetwork (LAN) card to provide a data communication connection to acompatible LAN. Wireless links may also be implemented. In any suchimplementation, communication interface 318 sends and receiveselectrical, electromagnetic or optical signals that carry digital datastreams representing various types of information.

Network link 320 typically provides data communication through one ormore networks to other data devices. For example, network link 320 mayprovide a connection through local network 322 to a host computer 324 orto data equipment operated by an Internet Service Provider (ISP) 326.ISP 326 in turn provides data communication services through the worldwide packet data communication network now commonly referred to as the“Internet” 328. Local network 322 and Internet 328 both use electrical,electromagnetic or optical signals that carry digital data streams. Thesignals through the various networks and the signals on network link 320and through communication interface 318, which carry the digital data toand from computer system 300, are exemplary forms of carrier wavestransporting the information.

Computer system 300 can send messages and receive data, includingprogram code, through the network(s), network link 320 and communicationinterface 318. In the Internet example, a server 330 might transmit arequested code for an application program through Internet 328, ISP 326,local network 322 and communication interface 318.

The received code may be executed by processor 304 as it is received,and/or stored in storage device 310, or other non-volatile storage forlater execution. In this manner, computer system 300 may obtainapplication code in the form of a carrier wave.

In the foregoing specification, embodiments of the invention have beendescribed with reference to numerous specific details that may vary fromimplementation to implementation. Thus, the sole and exclusive indicatorof what is the invention, and is intended by the applicants to be theinvention, is the set of claims that issue from this application, in thespecific form in which such claims issue, including any subsequentcorrection. Any definitions expressly set forth herein for termscontained in such claims shall govern the meaning of such terms as usedin the claims. Hence, no limitation, element, property, feature,advantage or attribute that is not expressly recited in a claim shouldlimit the scope of such claim in any way. The specification and drawingsare, accordingly, to be regarded in an illustrative rather than arestrictive sense.

What is claimed is:
 1. A method for handling deadline-based readrequests, the method comprising: in response to an arrival of a new readrequest having a first deadline, comparing (a) a first set of all datarequired to fulfill the new read request to (b) a second set of datathat will be retrieved to fulfill a previously-receivednot-yet-fulfilled read request, and comparing the first deadline to asecond deadline had by the previously-received not-yet-fulfilled readrequest; and determining, as a result of said comparing of sets of data,that the first set is completely included within the second set;determining, as a result of said comparing of deadlines, that the firstdeadline is not earlier than the second deadline; in response to anarrival of the second set of data and in response to determining thatthe first set is completely included within the second set, and inresponse to determining that the first deadline is not earlier than thesecond deadline, using the second set of data to fulfill both thepreviously-received not-yet-fulfilled read request and the new readrequest without performing separate read operations for the requests;wherein the method is performed by one or more computing devices.
 2. Themethod of claim 1, wherein the new read request comes from a firstclient, wherein the previously-received not-yet-fulfilled read requestcomes from a second client that is separate from the first client, andwherein said using of the second set of data to fulfill both therequests is performed by a read scheduler that issues requests to a filesystem on behalf of the first client and the second client.
 3. Themethod of claim 1, wherein said comparing of sets of data comprisescomparing the first set of data with sets of data required by requeststhat are contained in an in-flight list.
 4. The method of claim 1,wherein said comparing of sets of data comprises comparing the first setof data with sets of data required by requests that are contained in alist of requests that (1) have been issued to the file system but (2)have not been fulfilled for requesting clients.
 5. The method of claim1, wherein said comparing of deadlines comprises comparing the firstdeadline with deadlines had by requests that are contained in anin-flight list.
 6. The method of claim 1, wherein said comparing ofdeadlines comprises comparing the first deadline with deadlines had byrequests that are contained in a list of requests that (1) have beenissued to the file system but (2) have not been fulfilled for requestingclients.
 7. The method of claim 1, wherein said comparing of sets ofdata comprises comparing the first set of data with sets of datarequired by requests that are contained in an in-flight list, whereinsaid comparing of deadlines comprises comparing the first deadline withdeadlines had by requests that are contained in said in-flight list, andfurther comprising; in response to determining that the first set ofdata is included within a particular set of data required by aparticular request that is contained within the in-flight list, and inresponse to determining that the first deadline is not earlier than adeadline had by the particular request, fulfilling the new read requestusing the second set of data without adding the new read request to thein-flight list.
 8. The method of claim 1, wherein said comparing of setsof data comprises comparing the first set of data with sets of datarequired by requests that are contained in an in-flight list, whereinsaid comparing of deadlines comprises comparing the first deadline withdeadlines had by requests that are contained in said in-flight list, andfurther comprising: in response to determining, for every particularrequest within the in-flight list, that the first set of data is notincluded within a set of data required by the particular request,comparing said first set of data with sets of data required by requeststhat are within a list of previously-received read requests that havenot yet been moved to said in-flight list.
 9. The method of claim 1,wherein said comparing of sets of data comprises comparing the first setof data with sets of data required by requests that are contained in anin-flight list, wherein said comparing of deadlines comprises comparingthe first deadline with deadlines had by requests that are contained insaid in-flight list, and further comprising: in response to determining,for every particular request within the in-flight list, that the firstdeadline is later than a deadline had by the particular request,comparing said first deadline with deadlines had by requests that arewithin a list of previously-received read requests that have not yetbeen moved to said in-flight list.
 10. A non-transitorycomputer-readable storage medium storing instructions which, whenexecuted by one or more processors, cause the one or more processors toperform: in response to an arrival of a new read request having a firstdeadline, comparing (a) a first set of all data required to fulfill thenew read request to (b) a second set of data that will be retrieved tofulfill a previously-received not-yet-fulfilled read request, andcomparing the first deadline to a second deadline had by thepreviously-received not-yet-fulfilled read request; and determining, asa result of said comparing of sets of data, that the first set iscompletely included within the second set; determining, as a result ofsaid comparing of deadlines, that the first deadline is not earlier thanthe second deadline; in response to an arrival of the second set ofdata, and in response to determining that the first set is completelyincluded within the second set, and in response to determining that thefirst deadline is not earlier than the second deadline, using the secondset of data to fulfill both the previously-received not-yet-fulfilledread request and the new read request without performing separate readoperations for the requests; wherein the method is performed by one ormore computing devices.
 11. The non-transitory computer-readable storagemedium of claim 10, wherein the new read request comes from a firstclient, wherein the previously-received not-yet-fulfilled read requestcomes from a second client that is separate from the first client, andwherein said using of the second set of data to fulfill both therequests is performed by a read scheduler that issues requests to a filesystem on behalf of the first client and the second client.
 12. Thenon-transitory computer-readable storage medium of claim 10, whereinsaid comparing of sets of data comprises comparing the first set of datawith sets of data required by requests that are contained in anin-flight list.
 13. The non-transitory computer-readable storage mediumof claim 10, wherein said comparing of sets of data comprises comparingthe first set of data with sets of data required by requests that arecontained in a list of requests that (1) have been issued to the filesystem but (2) have not been fulfilled for requesting clients.
 14. Thenon-transitory computer-readable storage medium of claim 10, whereinsaid comparing of deadlines comprises comparing the first deadline withdeadlines had by requests that are contained in an in-flight list. 15.The non-transitory computer-readable storage medium of claim 10, whereinsaid comparing of deadlines comprises comparing the first deadline withdeadlines had by requests that are contained in a list of requests that(1) have been issued to the file system but (2) have not been fulfilledfor requesting clients.
 16. The non-transitory computer-readable storagemedium of claim 10, wherein said comparing of sets of data comprisescomparing the first set of data with sets of data required by requeststhat are contained in an in-flight list, wherein said comparing ofdeadlines comprises comparing the first deadline with deadlines had byrequests that are contained in said in-flight list, and wherein saidinstructions, when executed by said one or more processors, furthercause said one or more processors to perform: in response to determiningthat the first set of data is included within a particular set of datarequired by a particular request that is contained within the in-flightlist, and in response to determining that the first deadline is notearlier than a deadline had by the particular request, fulfilling thenew read request using the second set of data without adding the newread request to the in-flight list.
 17. The non-transitorycomputer-readable storage medium of claim 10, wherein said comparing ofsets of data comprises comparing the first set of data with sets of datarequired by requests that are contained in an in-flight list, whereinsaid comparing of deadlines comprises comparing the first deadline withdeadlines had by requests that are contained in said in-flight list, andwherein said instructions, when executed by said one or more processors,further cause said one or more processors to perform: in response todetermining, for every particular request within the in-flight list,that the first set of data is not included within a set of data requiredby the particular request, comparing said first set of data with sets ofdata required by requests that are within a list of previously-receivedread requests that have not yet been moved to said in-flight list. 18.The non-transitory computer-readable storage medium of claim 10, whereinsaid comparing of sets of data comprises comparing the first set of datawith sets of data required by requests that are contained in anin-flight list, wherein said comparing of deadlines comprises comparingthe first deadline with deadlines had by requests that are contained insaid in-flight list, and wherein said instructions, when executed bysaid one or more processors, further cause said one or more processorsto perform: in response to determining, for every particular requestwithin the in-flight list, that the first deadline is later than adeadline had by the particular request, comparing said first deadlinewith deadlines had by requests that are within a list ofpreviously-received read requests that have not yet been moved to saidin-flight list.